Test Of QCD Research Articles

Setting the renormalization scale in QCD: The principle of maximum conformality

A key problem in making precise perturbative QCD predictions is the uncertainty in determining the renormalization scale $\mu$ of the running coupling $\alpha_s(\mu^2).$ The purpose of the running coupling in any gauge theory is to sum all terms involving the $\beta$ function; in fact, when the renormalization scale is set properly, all non-conformal $\beta \ne 0$ terms in a perturbative expansion arising from renormalization are summed into the running coupling. The remaining terms in the perturbative series are then identical to that of a conformal theory; i.e., the corresponding theory with $\beta=0$. The resulting scale-fixed predictions using the "principle of maximum conformality" (PMC) are independent of the choice of renormalization scheme -- a key requirement of renormalization group invariance. The results avoid renormalon resummation and agree with QED scale-setting in the Abelian limit. The PMC is also the theoretical principle underlying the BLM procedure, commensurate scale relations between observables, and the scale-setting method used in lattice gauge theory. The number of active flavors $n_f$ in the QCD $\beta$ function is also correctly determined. We discuss several methods for determining the PMC scale for QCD processes. We show that a single global PMC scale, valid at leading order, can be derived from basic properties of the perturbative QCD cross section. The elimination of the renormalization scale ambiguity and the scheme dependence using the PMC will not only increase the precision of QCD tests, but it will also increase the sensitivity of collider experiments to new physics beyond the Standard Model.

Open charm and charmonium Production in high energy p+p and heavy-ion collisions

The overview of charm physics in p+p and heavy-ion collisions at RHIC energy is summarized. The latest results of open charm and charmonium production from RHIC-STAR experiment are presented, which are valuable for testing QCD theory via experiment, understanding the interactions between heavy quark and medium, exploring the different production mechanisms of open charm and charmonium. The measurements renew our knowledge of the fundamental properties of the QGP matter created in heavy-ion collisions.

Angular scaling in jets

Abstract We introduce a jet shape observable defined for an ensemble of jets in terms of two-particle angular correlations and a resolution parameter R. This quantity is infrared and collinear safe and can be interpreted as a scaling exponent for the angular distribution of mass inside the jet. For small R it is close to the value 2 as a consequence of the approximately scale invariant QCD dynamics. For large R it is sensitive to non-perturbative effects. We describe the use of this correlation function for tests of QCD, for studying underlying event and pile-up effects, and for tuning Monte Carlo event generators.

Angular correlations in three-jet events inepcollisions at HERA

Three-jet production in deep inelastic ep scattering and photoproduction was investigated with the ZEUS detector at HERA using an integrated luminosity of 127 pb-1. Measurements of differential cross sections are presented as functions of angular correlations between the three jets in the final state and the proton-beam direction. These correlations provide a stringent test of perturbative QCD and show sensitivity to the contributions from different colour configurations. Fixed-order perturbative QCD calculations assuming the values of the colour factors C_F, C_A and T_F as derived from a variety of gauge groups were compared to the measurements to study the underlying gauge group symmetry. The measured angular correlations in the deep inelastic ep scattering and photoproduction regimes are consistent with the admixture of colour configurations as predicted by SU(3) and disfavour other symmetry groups, such as SU(N) in the limit of large N.

Hadronic resonances from ALICE in pp collisions

The study of resonances in p s = 7 TeV pp collisions provides a test of QCD in a new energy domain as well as a baseline for heavy-ion collisions. The resonances K (892) 0 , (1020), (1385) , (1520), and (1530) 0 have been reconstructed at mid- rapidity from their hadronic decay using data collected by the ALICE detector. The com- parison of (1020) and (1385) pT-spectra to QCD-inspired models such as PHOJET and di erent PYTHIA tunes are shown.

Resonance results from the ALICE detector in 7 TeV pp collisions at the LHC

Author(s): Gangadharan, D | Abstract: The study of resonances in √s = 7 TeV pp collisions provides a test of QCD in a new energy domain as well as a baseline for heavy-ion collisions. The resonances K *(892)0, φ(1020), Λ(1520), Σ(1385) ±, and Ξ(1530) 0 are reconstructed at mid-rapidity from their hadronic decay using data collected by the ALICE detector. We compare the φ(1020) and Σ(1385) ± p T spectra to perturbative QCD models such as PHOJET and different PYTHIA tunes. We also search for the Φ(1860) pentaquark for which we find no evidence.

Heavy Flavour Production Measurements at the CERN-LHC

The study of heavy flavour (charm and beauty) production in proton-proton (pp) collisions at the LHC (Large Hadron Collider) is expected to provide an important test of perturbative QCD (pQCD) calculations in a new energy domain.1),2) In heavy-ion collisions, heavy quarks are, due to their large mass, produced in the early stage and experience the full evolution of the strongly-interacting medium expected to be formed in the collision. Heavy flavour particles should be sensitive to the density of this medium through the mechanism of in-medium energy loss of heavy quarks. This effect is usually studied by means of the nuclear modification factor:

GlueX: Photoproduction of Hybrid Mesons

The goal of the GlueX experiment is to provide critical data to help understand the soft gluonic field responsible for binding quarks in hadrons. Hybrid mesons, and in particular exotic hybrid mesons, provide the ideal laboratory for testing QCD in the confinement regime since these mesons explicitly manifest the gluonic degrees of freedom. Photoproduction is expected to be particularly effective in producing exotic hybrids but there is little data on the photoproduction of light mesons. GlueX will use the new 12-GeV electron beam to produce a 9-GeV beam of linearly polarized photons using the technique of coherent bremsstrahlung. A solenoid-based hermetic detector is under construction, which will be used to collect data on meson production and decays. These data will also be used to study the spectrum of conventional mesons, including the poorly understood excited vector mesons. This talk will describe the latest theoretical developments to help understand how the data of hybrid mesons can provide insights into the fundamental theory of strong interactions.

W/Z+jets with the CMS detector

We present a study of jet production in association with W and Z bosons in proton-proton collisions at TeV using the full 2010 data set collected by the CMS experiment corresponding to an integrated luminosity of 36±4 pb−1. The production of vector bosons with jets provides a stringent and important test of perturbative QCD, and is an important background to the study of top quark properties and in searches for new physics. We report the inclusive rates of jets produced (normalized to the total cross section) as well as the ratios σ(V + (n + 1)-jets)/σ(V + n-jets) for a jet threshold of 30 GeV. We also present a measurement of Z → ll + b and the first test of the Berends-Giele scaling at TeV.

Prospects of polarized fixed target Drell-Yan experiments

It has been proposed that the Siverse transverse single spin asymmetry in Drell-Yan production in transversely polarized p+p collisions would have an opposite sign compared to what has been observed in the polarized Semi-Inclusive Deep Inelastic Scattering (SIDIS) experiments. Experimental confirmation or disproval of this prediction would provide a novel fundamental test of QCD and shed new light on our theoretical understanding of the transverse spin physics phenomena. We discuss the prospects and physics sensitivities of polarized fixed target Drell-Yan experiments that could utilize the existing proton and other hadron beams at Fermilab, and polarized proton beams at RHIC with a polarized solid proton and/or neutron target option. We show that if realized, the new experiments would provide critical measurements of not only the sign change (or not) of Sivers functions, but also the information of quark and antiquark's Sivers distributions over a wide kinematic range.

Measurement of forward-rapidity heavy-flavour muons in proton-proton collisions at [formula omitted] with ALICE

We report the first results of the ALICE experiment on heavy-flavour measurements at forward 1 1 In this paper “forward” means in the LHC clockwise stream with respect to the interaction point. rapidity ( − 4 < η < − 2.5 ) in the muonic decay channel. The data have been collected during the proton-proton collisions at s = 7 TeV . In the ALICE program, the heavy-flavour detection in nucleon-nucleon interactions has a central role both as a test of perturbative QCD and as a baseline for the study of matter at high energy densities in heavy-ion collisions. The extraction of the heavy-flavour signal in the inclusive muon spectra requires the subtraction of the contribution from the decay-in-flight of light hadrons and from particles produced in the absorption elements. The details of the analysis and the main results will be discussed in this paper.

Measurement of $ {\text{B}}\overline {\text{B}} $ angular correlations based on secondary vertex reconstruction at $ \sqrt {s} = 7\,{\text{TeV}} $

A measurement of the angular correlations between beauty and anti-beauty hadrons (B B-bar) produced in pp collisions at a centre-of-mass energy of 7 TeV at the CERN LHC is presented, probing for the first time the region of small angular separation. The B hadrons are identified by the presence of displaced secondary vertices from their decays. The B hadron angular separation is reconstructed from the decay vertices and the primary-interaction vertex. The differential B B-bar production cross section, measured from a data sample collected by CMS and corresponding to an integrated luminosity of 3.1 inverse picobarns, shows that a sizable fraction of the B B-bar pairs are produced with small opening angles. These studies provide a test of QCD and further insight into the dynamics of b b-bar production.

Optimisation of variables for studying dilepton transverse momentum distributions at hadron colliders

In future measurements of the dilepton ($Z/\gamma^*$) transverse momentum, \Qt, at both the Tevatron and LHC, the achievable bin widths and the ultimate precision of the measurements will be limited by experimental resolution rather than by the available event statistics. In a recent paper the variable \at, which corresponds to the component of \Qt\ that is transverse to the dilepton thrust axis, has been studied in this regard. In the region, \Qt\ $<$ 30 GeV, \at\ has been shown to be less susceptible to experimental resolution and efficiency effects than the \Qt. Extending over all \Qt, we now demonstrate that dividing \at\ (or \Qt) by the measured dilepton invariant mass further improves the resolution. In addition, we propose a new variable, \phistarEta, that is determined exclusively from the measured lepton directions; this is even more precisely determined experimentally than the above variables and is similarly sensitive to the \Qt. The greater precision achievable using such variables will enable more stringent tests of QCD and tighter constraints on Monte Carlo event generator tunes.

Heavy Flavour Production at HERA

The production of hadrons containing a heavy quark is one of the most sensitive tests of QCD. At HERA, besides the investigation of production models incorporating multi-scale dependence, the production of heavy flavored hadrons is used to constrain the parton distribution functions, with important implications for LHC physics. Recent measurements by H1 and ZEUS experiments have been obtained using the HERA II data set and various tagging techniques for both photoproduction and Deep Inelastic Scattering. The results are in agreement with theoretical calculations. The DIS data have been combined recently to obtain a precise determination of the charm contribution to the proton structure function F 2 .

Charm content in jets in proton-proton collisions with the ALICE experiment at CERN-LHC

Charm and bottom quarks have been proposed as probes to study partonic matter produced in high-energy heavy-ion collisions. The detailed understanding of the production mechanisms in proton-proton collisions is of considerable interest as a QCD test tool and as reference calibration for heavy-ion studies. Measurements of the D meson yield in jets probe the production processes in which the observed D mesons are formed primarily from gluon splitting into cc̄ or bb̄ pairs. The charm content in jets is calculable in perturbative QCD, and the leading non-perturbative correction is expected to be significant at LHC energies. In this contribution we present a performance study of the reconstruction of charged D*± mesons in jets based on the first data collected by the ALICE experiment in minimum bias p+p collisions at = 7 TeV . D*+ mesons are reconstructed through the decay sequence D*+ → D0π+ and D0 → K−π+ (and its charge conjugate channel).

Physics Accomplishments of HERA

The electron-proton collider HERA at DESY in Hamburg, Germany, delivered luminosity from 1992 to 2007. The center-of-mass energy was a factor of ten higher compared to previous lepton-nucleon scattering experiments. The research at HERA emphasized precision analyses of the proton structure at small x and high Q2. In addition, precision tests of QCD provided significant new insights into the strong force. Diffractive scattering plays an important role in hard interactions. Many searches for new physics were performed at the electron-proton energy frontier. Additionally, two fixed-target experiments that used only the electron and proton beams of HERA, respectively, studied the spin structure of the nucleon and the production of strangeness, charm, and bottom in high-energy proton collisions. This review summarizes key results obtained by the four experiments at HERA: H1, ZEUS, HERMES, and HERA-B.

Formula omitted] + jet production at the Tevatron

Vector boson plus jet production is interesting for Higgs search, beyond the Standard Model physics and provides standard candles for calibration. This is complementary to inclusive jet production measurements which provide precision tests of perturbative QCD. A multitude of W/Z plus heavy and light flavour jet measurements in ppbar collisions at a centre of mass energy of sqrt{s}=1.96TeV is discussed. Next-to-Leading order perturbative QCD predictions and various models are compared to the measurements.

Determination of the strong coupling constant using available experimental data

We determine the strong coupling constant αs up to 4-loop in perturbative QCD. Testing QCD requires the measurement of αs over ranges of energy scales. In this analysis, the value of αs is determined from the unpolarized structure functions data points by minimizing the χ2 function between the theory result and experimental data. Using perturbative QCD calculations from threshold corrections, we obtain αs(M2Z) = 0.1139±0.0020 at N3LO which is in good agreement with the very recently results from the inclusive jet cross section in pp̄ collisions at √s = 1.96 TeV.

Quarkonium mass splittings in three-flavor lattice QCD

We report on calculations of the charmonium and bottomonium spectrum in lattice QCD. We use ensembles of gauge fields with three flavors of sea quarks, simulated with the asqtad improved action for staggered fermions. For the heavy quarks we employ the Fermilab interpretation of the clover action for Wilson fermions. These calculations provide a test of lattice QCD, including the theory of discretization errors for heavy quarks. We provide, therefore, a careful discussion of the results in light of the heavy-quark effective Lagrangian. By and large, we find that the computed results are in agreement with experiment, once parametric and discretization errors are taken into account.

QCD tests with an infrared finite gluon propagator and coupling constant

Recent progress in the solution of Schwinger-Dyson equations, as well as lattice simulation of pure glue QCD, indicate that the gluon propagator and coupling constant are infrared finite. Such non-perturbative information can be introduced in the QCD perturbative expansion in the scheme named Dynamical Perturbation Theory. We exemplify this procedure with the calculation of some two-body non-leptonic annihilation B meson decays, which show agreement with the experimental data in the case of a gluon propagator characterized by a dynamical gluon mass of 500MeV, compatible with the value found in several processes computed with this method. We give a preliminary account of the application of this procedure at the loop level in the case of the Bjorken sum rule.